Optical fiber for amplification and optical fiber amplifier using the same

ABSTRACT

An optical fiber for amplification includes a core having an inner core and an outer core surrounding the outer circumferential surface of the inner core. The relative refractive index difference of the inner core to a cladding is smaller than the relative refractive index difference of the outer core to the cladding. The outer core is entirely doped with erbium. The theoretical cutoff wavelength of an LP 11  mode light beam is a wavelength of 1,565 nm or more. The theoretical cutoff wavelength of an LP 21  mode light beam is a wavelength of 1,530 nm or less. The theoretical cutoff wavelength of the LP 02  mode light beam is a wavelength of 980 nm or less.

BACKGROUND OF THE INVENTION

The present invention relates to an optical fiber for amplification andan optical fiber amplifier using the same for use in opticalcommunications, which are suitable for the case of reducing thedifference in gains between an LP₀₁ mode light beam and an LP₁₁ modelight beam.

In optical communications, signals are superposed on light beamspropagating through an optical fiber for communications. In order toincrease information volumes transmitted through a single core in suchoptical communications, there are known few-mode communications in whichinformation is superposed on an LP₀₁ mode (a fundamental mode) lightbeam and on an LP₁₁ mode light beam of signal light beams forinformation communications. Therefore, in the case of amplifying signallight beams propagating through an optical fiber using an optical fiberamplifier, it is necessary to amplify both of the LP₀₁ mode and LP₀₂mode light beams.

Non Patent Literature 1 below describes an optical fiber foramplification in which the core of the optical fiber for amplificationhas a step refractive index, i.e., the refractive index of the core isconstant in the radial direction. The core is doped with erbium. Theconcentration of erbium is constant in the core. However, in the case ofamplifying light beams using such an optical fiber for amplification,the gain of the LP₀₁ mode light beam is often different from the gain ofthe LP₁₁ mode light beam.

Therefore, Non Patent Literature 2 below describes an optical fiber foramplification aiming for reducing such gain differences. The opticalfiber for amplification described in Non Patent Literature 2 has therefractive index profile of the core similar to the refractive indexprofile of the core of the optical fiber for amplification described inNon Patent Literature 1. However, in the optical fiber for amplificationin Non Patent Literature 2, erbium is not doped in the center regionincluding the center axis of the core, and erbium is doped in the outerregion surrounding the center region. Consequently, the power of theLP₀₁ mode light beam and the power of the LP₁₁ mode light beampropagating through the region doped with erbium are adjusted, and thusthe gain of the LP₀₁ mode light beam is made equal to the gain of theLP₁₁ mode light beam.

[Non Patent Literature 1] Y. Yung et al., “First demonstration ofmultimode amplifier for spatial division multiplexed transmissionsystems,” Proc. ECOC '11, Th.13.K4 (2011)

[Non Patent Literature 2] G. Le Cocq et al., “Modeling andcharacterization of few-mode EDFA supporting four mode groups for modedivision multiplexing,” Opt. Express 20, 27051-27061 (2012)

SUMMARY OF THE INVENTION

However, even in the optical fiber for amplification described in NonPatent Literature 2, the gain of the LP₀₁ mode light beam is sometimesvaried from the gain of the LP₁₁ mode light beam. This difference ingains between the LP₀₁ mode and LP₁₁ mode light beams is resulted fromthe fact that the pumping value of erbium is inconstant in the regiondoped with erbium. Pumping light to pump erbium propagates through thecore of the optical fiber for amplification as multi-mode light beams.Thus, in the region doped with erbium in the core, the power of pumpinglight is unevenly distributed. Consequently, in the region doped witherbium, the population inversion of erbium is inconstant. As describedabove, pumped erbium is unevenly distributed, causing a change in thegains of the LP₀₁ mode and LP₁₁ mode light beams even though the powerof the LP₀₁ mode light beam and the power of the LP₁₁ mode light beamare the same in signal light beams propagating through the region dopedwith erbium.

Consequently, it is important to adjust the power of a light beam ineach mode in pumping light propagating through the optical fiber foramplification, i.e., to adjust the excitation ratio of each mode inpumping light. However, since a complicated optical system is necessaryto adjust the excitation ratio, a more complicated optical system isnecessary as the number of modes is more increased. This creates demandsthat the number of modes of pumping light is decreased and thedifference in gains between the LP₀₁ mode and LP₁₁ mode light beams insignal light beams is easily reduced.

Therefore, it is an object of the present invention is to provide anoptical fiber for amplification and an optical fiber amplifier using thesame that can easily reduce the difference in gains between the LP₀₁mode and LP₁₁ mode light beams.

In order to solve the problem, an aspect of the present invention is anoptical fiber for amplification including a core having an inner coreand an outer core surrounding an outer circumferential surface of theinner core. The following is its features.

In other words, a relative refractive index difference of the inner coreto a cladding is smaller than a relative refractive index difference ofthe outer core to the cladding. The outer core is entirely doped witherbium. A theoretical cutoff wavelength of an LP₁₁ mode light beam is awavelength of 1,565 nm or more. A theoretical cutoff wavelength of anLP₂₁ mode light beam is a wavelength of 1,530 nm or less. A theoreticalcutoff wavelength of an LP₀₂ mode light beam is a wavelength of 580 nmor less.

According to the optical fiber for amplification of the embodiment ofthe present invention, in a waveband of 1,530 to 1,565 nm, i.e., in theC-band, the LP₀₁ mode and LP₁₁ mode light beams can be amplified.Accordingly, the optical fiber for amplification can be used for afew-mode communication optical fiber amplifier for signal light beams inthe C-band.

The relative refractive index difference of the inner core to thecladding is smaller than the relative refractive index difference of theouter core to the cladding. Thus, the power of the LP₀₁ mode light beamcan be unevenly distributed near to the outer core. Moreover, the outercore is doped with erbium. Consequently, in the case in which signallight beams propagate through the core, the difference in power betweenthe LP₀₁ mode and LP₁₁ mode light beams propagating through the regionof the core doped with erbium can be decreased. The theoretical cutoffwavelength of the LP₀₂ mode light beam is a wavelength of 980 nm orless. Thus, in the case in which pumping light to pump erbium propagatesthrough the core, the modes of the pumping light can be limited to theLP₀₁ mode, the LP₁₁ mode, the LP₂₁ mode, and an LP₃₁ mode. In Non PatentLiterature 2 above, there are six modes for pumping light. Compared withthis, the number of the modes of pumping light can be decreased.Consequently, the excitation ratio of light beams in these four modesonly has to be controlled so that the gain of the LP₀₁ mode light beamis equal to the gain of the LP₁₁ mode light beam in signal light beams.Therefore, according to the optical fiber for amplification of theembodiment of the present invention, the difference in gains between theLP₀₁ mode and LP₁₁ mode light beams can be easily reduced.

Further, a ratio D₁/D₂ is preferably 0.5 or greater, where a diameter ofthe inner core is defined as D₁, and an outer diameter of the outer coreis defined as D₂, and a ratio Δ_(nci)/Δ_(nco) is preferably 0.1 or less,where the relative refractive index difference of the inner core to thecladding is defined as Δ_(nci), the relative refractive index differenceof the outer core to the cladding is defined as Δ_(nco).

The core is thus configured. Consequently, in a predetermined range ofthe cutoff wavelength of the LP₂₁ mode light beam, the power can bealmost made equal between the LP₀₁ mode and LP₁₁ mode light beamspropagating through the outer core. Moreover, the theoretical cutoffwavelength of the LP₀₂ mode light beam can be more appropriately awavelength of 930 mn or less. From the definition of the inner core andthe outer core, the ratio D₁/D₂ is obviously smaller than one.

In this case, more preferably, the ratio D₁/D₂ is 0.6 or greater.

The ratio D₁/D₂ is set to 0.6 or greater. Thus, the difference in powerbetween the LP₀₁ mode and LP₁₁ mode light beams propagating through theouter core can be almost zero.

In this case, preferably, the ratio D₁/D₂ is 0.8 or less.

In a region in which the ratio D₁/D₂ is greater than 0.8, the differencebetween the sum total of the power of light in the LP₀₁ mode signallight beam propagating through the outer core and the sum total of thepower of light in the LP₁₁ mode signal light beam propagating throughthe outer core is not greatly changed, compared with the difference in aregion in which the ratio D₁/D₂ is 0.8 or less. On the other hand, underthe condition that the ratio D₁/D₂ is 0.8 or less, decreases in thegains of the LP₀₁ mode and LP₁₁ mode light beams in signal light beamscan be reduced.

In this case, preferably, the ratio Δ_(nci)/Δ_(nco) is zero or greater.

Preferably, the theoretical cutoff wavelength of the LP₂₁ mode lightbeam is a wavelength of 1,430 nm or more. More preferably, the cutoffwavelength is a wavelength of 1,450 nm or more.

An optical fiber amplifier according to an aspect of the presentinvention includes the optical fiber for amplification described above,and a pumping light source configured to emit a pumping light beam in a980 nm wavelength band, the pumping light beam being entered to thecore.

This optical fiber amplifier can reduce the difference in gains betweenthe LP₀₁ mode and LP₁₁ mode light beams in the C-band. Therefore, theoptical fiber amplifier more appropriately allows few-modecommunications in the C-band.

As described above, according to an aspect of the present invention,there are provided an optical fiber for amplification and an opticalfiber amplifier using the same that can easily reduce the difference ingains between the LP₀₁ mode and LP₁₁ mode light beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a cross section perpendicular to the longitudinaldirection of an optical fiber for amplification according to anembodiment of the present invention;

FIG. 2A is a diagram of a core and a cladding in a region indicated bydotted lines in FIG. 1;

FIG. 2B is a diagram of a refractive index profile in the region in FIG.2A;

FIG. 2C is a diagram of the concentration distribution of erbium dopedin the core;

FIG. 2D is a diagram of the power distribution of an LP₀₁ mode lightbeam and an LP₀₂ mode light beam propagating through the core;

FIG. 3 is a diagram of the relationship between the ratio of thediameter of the inner core to the outer diameter of the outer core andthe power of the LP₀₁ mode and LP₁₁ mode light beams propagating throughthe outer core;

FIG. 4 is a diagram of the relationship between the ratio between therelative refractive index difference of the inner core to the claddingand the relative refractive index difference of the outer core to thecladding and the power of the LP₀₁ mode and LP₁₁ mode light beamspropagating through the outer core;

FIG. 5 is a diagram of the relationship between the relative refractiveindex difference of the outer core to the cladding and the power of theLP₀₁ mode and LP₁₁ mode light beams propagating through the outer core;

FIG. 6 is a diagram of the relationship between modes of light beamspropagating at a wavelength of 1,550 nm and the radius of the inner coreand the outer radius of the outer core;

FIG. 7 is a diagram of the differential mode gains of the LP₀₁ mode andLP₁₁ mode light beams at points A to O in FIG. 6;

FIG. 8 is a diagram of the relationship between the difference in powerbetween the LP₀₁ mode and LP₁₁ mode light beams propagating through theouter core and the differential mode gains of the LP₀₁ mode and LP₁₁mode light beams;

FIG. 9 is a diagram of the relationship between the ratio of the powerbetween the LP₀₁ mode and LP₁₁ mode light beams propagating through theouter core and the differential mode gains of the LP₀₁ mode and LP₁₁mode light beams;

FIG. 10 is a diagram of the relationship between the difference in powerbetween the LP₀₁ mode and LP₁₁ mode light beams and the cutoffwavelength of an LP₂₁ mode light beam;

FIG. 11 is a diagram of the relationship between the ratio of thediameter of the inner core to the outer diameter of the outer core andthe cutoff wavelength;

FIG. 12 is a diagram of the relationship between the ratio between therelative refractive index difference of the inner core to the claddingand the relative refractive index difference of the outer core to thecladding and the cutoff wavelength; and

FIG. 13 is a diagram of an optical fiber amplifier according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a preferred embodiment of an optical fiber foramplification according to an embodiment of the present invention and anoptical fiber amplifier using the same will be described in detail withreference to the drawings. For easy understanding, the scales of thedrawings are sometimes different from the scales in the followingdescription.

Description of Amplification Optical Fiber

FIG. 1 is a diagram of a cross section perpendicular to the longitudinaldirection of an optical fiber for amplification according to anembodiment of the present invention. As illustrated in FIG. 1, anoptical fiber for amplification 10 includes a core 11, a cladding 12surrounding the outer circumferential surface of the core 11 with nogap, and a buffer layer 14 covering the cladding 12 as main components.The diameter of the core 11 is 10 μm, for example. The outer diameter ofthe cladding 12 is 125 μm, for example.

The optical fiber for amplification 10 according to the embodiment is afew-mode fiber through which an LP₀₁ mode light beam and an LP₀₂ modelight beam in the C-band are transmitted. In other words, in the opticalfiber for amplification 10, the theoretical cutoff wavelength of theLP₁₁ mode light beam is a wavelength of 1,565 nm or more, and thetheoretical cutoff wavelength of an LP₂₁ mode light beam is a wavelengthof 1,530 nm or less.

FIGS. 2A to 2D are diagrams of the core 11 of the optical fiber foramplification 10 in FIG. 1 and regions around the core 11. Morespecifically, FIG. 2A is a diagram of the core 11 and the cladding 12 inthe region indicated by dotted lines in FIG. 1. FIG. 2B is a diagram ofthe refractive index profile in the regions in FIG. 2A. FIG. 2C is adiagram of the concentration distribution of erbium doped in the core11. FIG. 2D is a diagram of the power distribution of the LP₀₁ mode andLP₀₂ mode light beams propagating through the core 11.

As illustrated in FIG. 2A, the core 11 is formed of an inner core 11 iincluding the center axis and an outer core 11 o surrounding the outercircumferential surface of the inner core 11 i with no gap.

As illustrated in FIG. 2B, the refractive index of the inner core 11 iis lower than the refractive index of the outer core 11 o. In theembodiment, the refractive index of the inner core 11 i is equal to therefractive index of the cladding 12. The relative refractive indexdifference of the outer core 11 o to the cladding 12 is 1%, for example.Since such a refractive index profile is provided, for example, theouter core 11 o is made of silica doped with a dopant such as germanium(Ge) to increase the refractive index. The inner core 11 i and thecladding 12 are formed of silica doped with no dopant. In the case inwhich the outer core 11 o is made of silica doped with no dopant, theinner core 11 i and the cladding 12 are made of silica doped with adopant such as fluorine to decrease the refractive index.

As illustrated in FIG. 2C, the outer core 11 o is doped with erbium. Inthe embodiment, the outer core 11 o is entirely doped with erbium,whereas the inner core 11 i is doped with no erbium.

The optical fiber for amplification 10 transmits light beams in fewmodes, the LP₀₁ mode and the LP₁₁ mode, in the C-band. In the case inwhich the refractive index of the core 11 is constant in the radialdirection, the peak of the power of the LP₀₁ mode light beam isoriginally located in the center of the core. However, since the core 11of the optical fiber for amplification 10 according to the embodimenthas the refractive index profile as illustrated in FIG. 2B, light beamspropagating through the core are displaced to the outer circumferentialside. Consequently, as illustrated in FIG. 20, the LP₀₁ mode light beamis also displaced to the outer circumferential side, and the peak of thepower of the LP₀₁ mode light beam is located as displaced from thecenter to the outer circumferential side. The LP₁₁ mode light beam isalso displaced to the outer circumferential side. In the embodiment, inlight beams in the C-band propagating through the core 11, the sum totalof the power of light in the LP₀₁ mode light beam propagating throughthe outer core 11 o is made almost equal to the sum total of the powerof light in the LP₁₁ mode light beam propagating through the outer core11 o.

Next, a configuration will be described in which the sum total of thepower of the LP₀₁ mode light beam is almost equal to the sum total ofthe power of the LP₁₁ mode light beam in the outer core 11 o.

FIG. 3 is a diagram of the relationship between a ratio of a diameter D₁of the inner core 11 i to an outer diameter D₂ of the outer core 11 o(the diameter of the core 11) and the power of the LP₀₁ mode and LP₁₁mode light beams propagating through the outer core 11 o. In FIG. 3, thesum total of the power of light in the LP₀₁ mode light beam propagatingthrough the outer core 11 o is expressed by Γ₀₁, and the sum total ofthe power of light in the LP₁₁ mode light beam propagating through theouter core 11 o is expressed by Γ₁₁. The difference between the powerΓ₀₁ and the power Γ₁₁ is expressed by ΔΓ. The vertical axis expressesthe values of the power Γ₀₁ , the power Γ₁₁, and the difference ΔΓ. Thehorizontal axis expresses the ratio D₁/D₂ of the diameter D₁ of theinner core 11 i to the outer diameter D₂ of the outer core 11 o. In FIG.3, the cutoff wavelength of the LP₂₁ mode light beam is set to awavelength of 1,450 nm. The relative refractive index difference of theouter core 11 o to the cladding 12 is set to 1%. The refractive index ofthe inner core 11 i is equal to the refractive index of the cladding.Under these conditions, the outer diameter D₂ is defined. As illustratedin FIG. 3, under the condition that the ratio D₁/D₂ is 0.5 or greater,almost no difference is present between the values of the power Γ₀₁ andthe power Γ₁₁, and the absolute value of the difference ΔΓ is 0.01 orless. Under the condition that the ratio D₁/D₂ is 0.6 or greater, thedifference ΔΓ between the values of the power Γ₀₁ and the power Γ₁₁ isalmost zero. Preferably, the upper limit of the ratio D₁/D₂ is 0.8 sothat the gains of the LP₀₁ mode and LP₁₁ mode light beams are not toosmall because of too small vales of the power Γ₀₁ and the power Γ₁₁. InFIG. 3, the wavelength of light beams propagating through the core 21 isset to a wavelength of 1,550 nm. However, the tendency observed in FIG.3 is similarly observed at other wavelengths. As described above, thecutoff wavelength of the LP₂₁ mode light beam is set to a wavelength of1,450 nm. Consequently, under the condition that the cutoff wavelengthof the LP₂₁ mode light beam is at least a wavelength of 1,450 nm, theabove-described tendency is observed in the ratio D₁/D₂. However, asdescribed later, even though the cutoff wavelength of the LP₂₁ modelight beam is changed by about 20 nm from the cutoff wavelength in FIG.3, the tendency is not different so much from FIG. 3. Therefore, underthe condition that the cutoff wavelength of the LP₂₁ mode light beam isa wavelength of 1,430 nm, the tendency is not different so much fromFIG. 3.

Next, in the case in which the ratio D₁/D₂ is 0.5, a ratioΔ_(nci)/Δ_(nco) between a relative refractive index difference Δ_(nci)of the inner core 11 i to the cladding 12 and a relative refractiveindex difference Δ_(nco) of the outer core 11 o to the cladding 12 ischanged. FIG. 4 is a diagram of the relationship of the ratioΔ_(nci)/Δ_(nco) to the power Γ₀₁ of the LP₀₁ mode light beam and thepower Γ₁₁ of the LP₁₁ mode light beam propagating through the outer core11 o. As illustrated in FIG. 4, under the condition that the ratioΔ_(nci)/Δ_(nco) is 0.1 or less, the value of the power Γ₀₁ is notdifferent from the value of the power Γ₁₁. The conditions of thewavelength of light propagating through the core 11 in FIG. 4 are thesame as the conditions of the wavelength of light propagating throughthe core 11 in FIG. 3. The tendency observed in FIG. 4 is almost thesame at other wavelengths. The conditions of the cutoff wavelength ofthe LP₂₁ mode light beam in FIG. 4 are the same as the conditions of thecutoff wavelength of the LP₂₁ mode light beam in FIG. 3. Consequently,under the condition that the cutoff wavelength of the LP₂₁ mode lightbeam is at least a wavelength of 1,450 nm, the tendency of the ratioΔ_(nci)/Δ_(nco) is observed. Even though the cutoff wavelength of theLP₂₁ mode light beam is changed by about 20 nm from FIG. 4, the tendencyis not different so much from FIG. 4. Consequently, under the conditionthat the cutoff wavelength of the LP₂₁ mode light beam is a wavelengthof 1,430 nm, the tendency is not different so much from FIG. 4.

FIG. 5 is a diagram of the relationship between the relative refractiveindex difference Δ_(nco) of the outer core 11 o to the cladding 12 andthe power Γ₀₁ of the LP₀₁ mode light beam and the power Γ₁₁ of the LP₁₁mode light beam propagating through the outer core 11 o. In FIG. 5, theratio D₁/D₂ of the diameter D₁ of the inner core 11 i to the outerdiameter D₂ of the outer core 11 o is 0.4. The relative refractive indexdifference Δ_(nci) of the inner core 11 i to the cladding is zero. Theouter core 11 o has the outer diameter D₂ in which the cutoff wavelengthof the LP₂₁ mode light beam is a wavelength of 1,450 nm. Under theseconditions, the horizontal axis expresses the relative refractive indexdifference Δ_(nco) of the outer core 11 o to the cladding, and thevertical axis expresses the power Γ₀₁ of the LP₀₁ mode light beam andthe power Γ₁₁ of the LP₁₁ mode light beam propagating through the outercore 11 o. From FIG. 6, the difference ΔΓ between Γ₀₁ and Γ₁₁ isconstant regardless of Δ_(nco). In other words, even in the case inwhich the refractive index of the outer core 11 o is changed, thischange gives no influence on the difference ΔΓ.

From FIGS. 3 to 5, under the conditions that the ratio D₁/D₂ is 0.5 orgreater and the ratio Δ_(nci)/Δ_(nco) is 0.1 or less, the power Γ₀₁ ofthe LP₀₁ mode light beam is almost equal to the power Γ₁₁ of the LP₁₁mode light beam in the outer core 11 o.

Next, referring to FIGS. 6 to 10, the relationship between thedifferential mode gain (DMG) and the cutoff wavelength will bedescribed.

FIG. 6 is a diagram of the relationship of the modes of light beamspropagating at a wavelength of 1,550 nm, a radius R_(i) of the innercore 11 i, and an outer radius R_(o) of the outer core 11 o. In FIG. 6,the wavelength of light beams propagating through the core 11 was set toa wavelength of 1,550 nm. The relative refractive index differenceΔ_(nci) of the inner core 11 i to the cladding 12 was set to 0%. Therelative refractive index difference Δ_(nco) of the outer core 11 o tothe cladding 12 was set to 1%. In FIG. 6, the relative refractive indexdifferences are fixed, and the radius R_(i) of the inner core 11 i andthe outer radius R_(o) of the outer core 11 o are changed as describedabove. Thus, in FIG. 6, the cutoff wavelength of the LP₂₁ mode lightbeam is inconstant.

In FIG. 6, in a region AR_(N), the diameter of the inner core 11 i isgreater than the outer diameter of the outer core 11 o. The regionAR_(N) is not physically possible. In a region AR₀, light beams in anymodes do not propagate. In a region AR₁, only the LP₀₁ mode light beampropagates. In a region AR₂, only the LP₀₁ mode and LP₁₁ mode lightbeams propagate. In a region AR₃, only the LP₀₁ mode light beam, theLP₁₁ mode light beam, and the LP₂₁ mode light beam propagate. In aregion AR₄, only the LP₀₁ mode light beam, the LP₁₁ mode light beam, theLP₂₁ mode light beam, and the LP₀₂ mode light beam propagate.Consequently, for the relationship between the radius R_(i) of the innercore 11 i and the outer radius R_(o) of the outer core 11 o of theoptical fiber for amplification 10 according to the embodiment, theradius R_(i) and the outer radius R_(o) have to be included in theregion AR₂.

Next, differential mode gains at points in the region AR₂ will bedescribed. FIG. 7 is a diagram of the differential mode gains of theLP₀₁ mode and LP₁₁ mode light beams at points A to O in the region AR₂in FIG. 6. FIG. 8 is a diagram of the relationship between thedifference of the power Γ₀₁ of the LP₀₁ mode light beam to the power Γ₁₁of the LP₁₁ mode light beam propagating through the outer core 11 o(Γ₀₁-Γ₁₁) and the differential mode gains of the LP₀₁ mode and LP₁₁ modelight beams. FIG. 9 is a diagram of the relationship between the ratioof the power Γ₀₁ of the LP₀₁ mode light beam to the power Γ₁₁ of theLP₁₁ mode light beam propagating through the outer core 11 o (Γ₁₁/Γ₀₁)and the differential mode gains of the LP₀₁ mode and LP₁₁ mode lightbeams. In FIGS. 7 to 9, the conditions of the wavelengths of light beamspropagating through the core 11, the relative refractive indexdifference Δ_(nci) of the inner core 11 i to the cladding 12, and therelative refractive index difference Δ_(nco) of the outer core 11 o tothe cladding 12 were the same as the conditions in FIG. 6. In FIGS. 7 to9, the differential mode gain caused by pumping light in the LP₀₁ mode,the differential mode gain caused by pumping light in the LP₁₁ mode, andthe differential mode gain caused by pumping light in the LP₂₁ mode areseparately depicted.

As illustrated in FIG. 7, at points E and F, the differential mode gainsare very small. At points E and F, the difference Γ₀₁−Γ₁₁(=ΔΓ) in FIG. 8is plotted approximately at zero, and the ratio Γ₁₁/Γ₀₁ in FIG. 9 isplotted approximately at one. From FIG. 6, at points E and F, theconditions are satisfied in which the ratio D₁/D₂ is 0.5 or greater andthe ratio Δ_(nci)/Δ_(nco) is 0.1 or less. As described above, in FIG. 6,the relative refractive index difference Δ_(nci) is set to 0%, and therelative refractive index difference Δ_(nco) is set to 1%. From FIGS. 7to 9, at points E and F, the difference Γ₀₁−Γ₁₁(=ΔΓ) is plottedapproximately at zero, and the differential mode gains are very small.On the other hand, in FIG. 6, although the conditions are satisfied inwhich the ratio D₁/D₂ is 0.5 or greater and the ratio Δ_(nci)/Δ_(nco) is0.1 or less at some points, the differential mode gain is not small somuch like point H. At such points, the cutoff wavelength of the LP₂₁mode light beam is much smaller than the cutoff wavelength (1,450 nm) inFIGS. 3 and 4.

FIG. 10 is a diagram of the relationship between the difference ΔΓ ofthe power Γ₀₁ of the LP₀₁ mode light beam to the power Γ₁₁ of the LP₂₁mode light beam and a cutoff wavelength λc (LP₂₁) of the LP₂₁ mode lightbeam. In FIG. 10, the ratio D₁/D₂ was set to 0.5. The relativerefractive index difference Δ_(nci) was set to 0%. The relativerefractive index difference Δ_(nco) was set to 1%. The diameter D₂ wasthen changed from 6.2 μm to 9.4 μm, and the cutoff wavelength of theLP₂₁ mode light beam was changed as illustrated in FIG. 10.

As illustrated in FIG. 10, the difference ΔΓ is smaller as the cutoffwavelength of the LP₂₁ mode light beam is a longer wavelength. In otherwords, in FIGS. 3 and 4, the cutoff wavelength of the LP₂₁ mode lightbeam is a wavelength of 1,450 nm. However, under the condition that thecutoff wavelength is longer than a wavelength of 1,450 nm, thedifference ΔΓ becomes much smaller. From FIG. 10, under the conditionthat the cutoff wavelength of the LP₂₁ mode light beam is a wavelengthof 1,430 nm or more, the difference ΔΓ is 0.01 or less. Consequently, asdescribed in FIGS. 3 and 4, under the condition that the cutoffwavelength of the LP₂₁ mode light beam is a wavelength of 1,430 nm, thetendency is not different so much from FIGS. 3 and 4. In other words,the conditions are satisfied in which the cutoff wavelength of the LP₂₁mode light beam is a wavelength of 1,430 nm, the ratio D₁/D₂ is 0.5 orgreater, and Δ_(nci)/Δ_(nco) is 0.1 or less. Thus, the difference ΔΓ canbe set to 0.01 or less.

Next, a configuration will be described in which the cutoff wavelengthof the LP₀₂ mode light beam propagating through the core 11 is smallerthan a wavelength of 980 nm.

FIG. 11 is a diagram of the relationship between the ratio D₁/D₂ of thediameter D₁ of the inner core 11 i to the outer diameter D₂ of the outercore 11 o and the cutoff wavelength λc. In FIG. 11, the cutoffwavelength of the LP₂₁ mode light beam is set to a wavelength of 1,500nm. The relative refractive index difference of the outer core 11 o tothe cladding 12 is set to 1%. The refractive index of the inner core 11i is the same as the cladding. Under these conditions, the outerdiameter D₂ is defined. In FIG. 11, the relationship among the LP₁₁ modelight beam, the LP₂₁ mode light beam, the LP₀₂ mode light beam, an LP₃₁mode light beam, and the LP₁₂ mode light beam is illustrated. From FIG.11, under the condition that the ratio D₁/D₂ is 0.5 or greater, thecutoff wavelengths of the LP₀₂ mode and LP₁₂ mode light beams aresmaller than a wavelength of 980 nm. Thus, under the condition that theratio D₁/D₂ is 0.5 or greater, the modes of the pumping light at awavelength of 980 nm can be the LP₀₁ mode, the LP₁₁ mode, the LP₂₁ mode,and the LP₃₁ mode.

Next, in the case in which the ratio D₁/D₂ is 0.5, the ratioΔ_(nci)/Δ_(nco) between the relative refractive index difference Δ_(nci)of the inner core 11 i to the cladding 12 and the relative refractiveindex difference Δ_(nco) of the outer core 11 o to the cladding 12 ischanged. FIG. 12 is a diagram of the relationship between the ratioΔ_(nci)/Δ_(nco) and the cutoff wavelength λc. As illustrated in FIG. 12,under the condition that the ratio λ_(nci)/Δ_(nco) is 0.1 or less, thecutoff wavelengths of the LP₀₂ mode and LP₁₂ mode light beams aresmaller than a wavelength of 980 nm. Thus, under the condition that theratio Δ_(nci)/Δ_(nco) is 0.1 or less, the modes of the pumping light ata wavelength of 960 nm can be the LP₀₁ mode, the LP₁₁ mode, the LP₂₁mode, and the LP₃₁ mode.

In other words, from FIGS. 11 and 12, under the conditions that theratio D₁/D₂ is 0.5 or greater and the ratio Δ_(nci)/Δ_(nco) is 0.1 orless, the modes of the pumping light at a wavelength of 980 nm to pumperbium doped to the outer core 11 o can be the LP₀₁ mode, the LP₁₁ mode,the LP₂₁ mode, and the LP₃₁ mode.

As described above, in the optical fiber for amplification 10, thetheoretical cutoff wavelength of the LP₁₁ mode light beam is awavelength of 1,565 nm or more, and the theoretical cutoff wavelength ofthe LP₂₁ mode light beam is a wavelength of 1,530 nm or less. Forexample, in order to achieve the theoretical cutoff wavelengthsdescribed above under the conditions that the ratio D₁/D₂ is 0.5, theratio Δ_(nci)/Δ_(nco) is 0, and the relative refractive index differenceΔ_(nco) is 1.0%, the relationship 6.2 μm≦D₂≦9.4 μm only has to be held.

As described above, according to the optical fiber for amplification 10of the embodiment, the LP₀₁ mode and LP₁₁ mode light beams can beamplified in the C-band with the LP₀₁ mode and LP₁₁ mode light beamspropagating through the optical fiber for amplification 10. Thedifference in power between the LP₀₁ mode and LP₁₁ mode light beamspropagating through the outer core 11 o can be decreased. The outer core11 o is doped with erbium. Thus, the difference in power between theLP₀₁ mode and LP₁₁ mode light beams propagating through the region dopedwith erbium can be decreased. Therefore, the appropriate adjustment ofthe population inversion of erbium can reduce the difference in gainsbetween the LP₀₁ mode and LP₁₁ mode light beams.

In the optical fiber for amplification 10 according to the embodiment,the theoretical cutoff wavelength of the LP₀₂ mode light beam is awavelength of 980 nm or less. Thus, in the case in which the pumpinglight to pump erbium propagates through the core, the modes of thepumping light can be limited to the LP₀₁ mode, the LP₁₁ mode, the LP₂₁mode, and the LP₃₁ mode. Therefore, the excitation ratio of light amongthese four modes only has to be controlled so that the gain of the LP₀₁mode light beam is equal to the gain of the LP₁₁ mode light beam insignal light beams. Accordingly, according to the optical fiber foramplification 10 of the embodiment, the difference in gains between theLP₀₁ made and LP₁₁ mode light beams can be easily reduced.

Description of the Optical Fiber Amplified

Next, an optical fiber amplifier using the optical fiber foramplification 10 will be described with reference to FIG. 13.

FIG. 13 is a diagram of an optical fiber amplifier according to theembodiment. As illustrated in FIG. 13, an optical fiber amplifier 1according to the embodiment includes, as main components, an opticalfiber 21 through which signal light beams propagate for amplification,an optical isolator 30 a provided in the midway point of the opticalfiber 21, a WDM coupler 40 a connected to the optical fiber 21, anoptical fiber 22 having one end connected to the WDM coupler 40 a, theoptical fiber for amplification 10 having one end connected to the otherend of the optical fiber 22, an optical fiber 24 having one endconnected to the other end of the optical fiber for amplification 10, aWDM coupler 40 b connected to the other end of the optical fiber 24, anoptical fiber 25 connected to the WDM coupler 40 b, an optical isolator30 b provided in the midway point of the optical fiber 25, and a pumpinglight source 50.

The optical fiber 21 is a few-mode fiber. The LP₀₁ mode and LP₁₁ modelight beams in the C-band, which are signal light beams, propagatethrough the optical fiber 21. In optical fiber 21, signals aresuperposed on the LP₀₁ mode and LP₁₁ mode light beams. These light beamspropagate through the optical fiber 21 toward the WDM coupler 40 a.

The optical isolator 30 a provided in the midway point of the opticalfiber 21 transmits the signal light beams propagating from the opticalfiber 21 to the WDM coupler 40 a, and blocks the transmission of lightbeams propagating toward the opposite side. For example, light isunnecessarily reflected inside the optical fiber amplifier 1 and travelsin the direction opposite to the traveling direction of the signal lightbeams. The optical isolator 30 a blocks the entrance of the reflectedlight from the optical isolator 30 a to the optical fiber 21.

The pumping light source 50 emits pumping light at a wavelength of 980nm. From the pumping light emitted from the pumping light source 50, theLP₀₁ mode light beam, the LP₁₁ mode light beam, the LP₂₁ mode lightbeam, and the LP₃₁ mode light beam are individually emitted. Forexample, other than the LP₀₁ mode light beam, LP₀₁ mode light beams tobe the sources of the LP₁₁ mode light beam, the LP₂₁ mode light beam,and the LP₃₁ mode light beam are individually emitted, and then the LP₁₁mode light beam, the LP₂₁ mode light beam, and the LP₃₁ mode light beamare individually excited from these light beams. A wave phase plate onlyhas to be used for excitation. The excited light beams in the modes areindividually emitted, and then individually entered to the WDM coupler40 a. This is the configuration of the pumping light source 50. In orderto adjust the power of each of the light beams in the modes, the powerof each of the LP₀₁ mode light beams to be the sources of the lightbeams in the modes only has to be adjusted.

To the WDM coupler 40 a, the signal light beams are entered from theoptical fiber 21, and the pumping light beams are entered from thepumping light source 50. The WDM coupler 40 a multiplexes the signallight beams with the pumping light beams, which have been entered, andenters the multiplexed light beams to the optical fiber 22. The opticalfiber 22 is configured similarly to the optical fiber 21.

In the optical fiber for amplification 10 connected to the optical fiber22, the core 11 satisfies the conditions that the ratio D₁/D₂ is 0.5 orgreater and the ratio Δ_(nci)/Δ_(nco) is 0.1 or less. To the opticalfiber for amplification 10, the LP₀₁ mode and LP₁₁ mode light beams inthe C-band propagating through the optical fiber 21 are entered, and thepumping light beams at a wavelength of 980 nm emitted from the pumpinglight source are entered. In the signal light beams entered to theoptical fiber for amplification 10 and transmitted through the core 11,the power Γ₀₁ of the LP₀₁ mode light beam is almost equal to the powerΓ₁₁ of the LP₁₁ mode light beam in the outer core 11 o under theconditions satisfied on the core 11. On the other hand, the modes of thepumping light beams entered to the optical fiber for amplification 10and transmitted through the core 11 are the LP₀₁ mode, the LP₁₁ mode,the LP₂₁ mode, and the LP₃₁ mode under the conditions satisfied on thecore 11. The pumping light beams in these modes then pump erbium dopedin the outer core 11 o. The pumped erbium causes stimulated emissionwith the signal light beams for amplifying the signal light beams.

At this amplifying, as described above, the power Γ₀₁ of the LP₀₁ modelight beam is almost equal to the power Γ₁₁ of the LP₁₁ mode light beamin the outer core 11 o. The excitation of the pumping light beams inthese four modes is adjusted. Consequently, the LP₀₁ mode and LP₁₁ modelight beams in the signal light beams have almost the same gains. Inorder to achieve such gains, for example, the power of each of thepumping light beams in four modes is compared with the power of the LP₀₁mode and LP₁₁ mode light beams in the signal light beams emitted fromthe optical fiber for amplification 10. The power of each of the pumpinglight beams in four modes is adjusted so that the gain of the LP₀₁ modelight beam is almost equal to the gain of the LP₁₁ mode light beam inthe signal light beams.

The signal light beams including the LP₀₁ mode and LP₁₁ mode light beamsalmost equally amplified are emitted from the optical fiber foramplification 10.

The optical fiber 24 connected to the optical fiber for amplification 10is configured similarly to the optical fiber 22. The signal light beamsand the excess pumping light beams emitted from the optical fiber foramplification 10 are entered to the optical fiber 24, and transmittedthrough the optical fiber 24.

The signal light beams and the excess pumping light beams entered fromthe optical fiber 24 to the WDM coupler 40 b are separated at the WDMcoupler 40 b. The separated excess pumping light beams are lost at aterminating device E. The signal light beams are entered to the opticalfiber 25, and transmitted through the optical fiber 25.

The optical isolator 30 b provided in the midway point of the opticalfiber 25 transmits the signal light beams propagating from the WDMcoupler 40 b through the optical fiber 25, and blocks the transmissionof light beams propagating toward the WDM coupler 40 b. Consequently,the signal light beams are transmitted and emitted from the opticalisolator 30 b.

According to the optical fiber amplifier 1 of the embodiment, theoptical fiber for amplification 10 reduces the difference in gainsbetween the LP₀₁ mode and LP₁₁ mode light beams. Thus, light beams infew modes with small gain differences can be emitted.

As described so far, the embodiment is taken as an example fordescribing the present invention. The present invention is not limitedto the embodiment.

For example, in FIG. 2, the relative refractive index difference of theinner core 11 i to the cladding 12 is set to 0%. However, under thecondition that the theoretical cutoff wavelength of the LP₀₂ mode lightbeam is a wavelength of 980 nm or less, the relative refractive indexdifference has any percentage. However, preferably, the ratioΔ_(nci)/Δ_(nco) is 0.1 or less as described above.

As described above, according to the embodiment of the presentinvention, there is provided an optical fiber for amplification and anoptical fiber amplifier using the same that can easily reduce thedifference in gains between the LP₀₁ mode and LP₁₁ mode light beams. Theoptical fiber for amplification and the optical fiber amplifier areexpected for use in the field of few-mode optical communications.

1. An optical fiber for amplification comprising: a core having an innercore and an outer core surrounding an outer circumferential surface ofthe inner core, wherein a relative refractive index difference of theinner core to a cladding is smaller than a relative refractive indexdifference of the outer core to the cladding, the outer core is entirelydoped with erbium, a theoretical cutoff wavelength of an LP₁₁ mode lightbeam is a wavelength of 1,565 nm or more, a theoretical cutoffwavelength of an LP₂₁ mode light beam is a wavelength of 1,530 nm orless, and a theoretical cutoff wavelength of an LP₀₂ mode light beam isa wavelength of 980 nm or less.
 2. The optical fiber for amplificationaccording to claim 1, wherein a ratio D₁/D₂ is 0.5 or greater, where adiameter of the inner core is defined as D₁, and an outer diameter ofthe outer core is defined as D₂, and a ratio Δ_(nci)/Δ_(nco) is 0.1 orless, where the relative refractive index difference of the inner coreto the cladding is defined as Δ_(nci), and the relative refractive indexdifference of the outer core to the cladding is defined as Δ_(nco). 3.The optical fiber for amplification according to claim 2, wherein theratio D₁/D₂ is 0.6 or greater.
 4. The optical fiber for amplificationaccording to claim 2, wherein the ratio D₁/D₂ is 0.8 or less.
 5. Theoptical fiber for amplification according to claim 2, wherein the ratioΔ_(nci)/Δ_(nco) is zero or greater.
 6. The optical fiber foramplification according to claim 2, wherein the theoretical cutoffwavelength of the LP₂₁ mode light beam is a wavelength of 1,430 nm ormore.
 7. The optical fiber for amplification according to claim 6,wherein the theoretical cutoff wavelength of the LP₂₁ mode light beam isa wavelength of 1,450 nm or more.
 8. The optical fiber for amplificationaccording to claim 3, wherein the ratio D₁/D₂ is 0.8 or less.
 9. Theoptical fiber for amplification according to claim 3, wherein the ratioΔ_(nci)/Δ_(nco) is zero or greater.
 10. The optical fiber foramplification according to claim 3, wherein the theoretical cutoffwavelength of the LP₂₁ mode light beam is a wavelength of 1,430 nm ormore.
 11. The optical fiber for amplification according to claim 10,wherein the theoretical cutoff wavelength of the LP₂₁ mode light beam isa wavelength of 1,450 nm or more.
 12. The optical fiber foramplification according to claim 4, wherein the ratio Δ_(nci)/Δ_(nco) iszero or greater.
 13. The optical fiber for amplification according toclaim 4, wherein the theoretical cutoff wavelength of the LP₂₁ modelight beam is a wavelength of 1,430 nm or more.
 14. The optical fiberfor amplification according to claim 13, wherein the theoretical cutoffwavelength of the LP₂₁ mode light beam is a wavelength of 1,450 nm ormore.
 15. The optical fiber for amplification according to claim 5,wherein the theoretical cutoff wavelength of the LP₂₁ mode light beam isa wavelength of 1,430 nm or more.
 16. The optical fiber foramplification according to claim 15, wherein the theoretical cutoffwavelength of the LP₂₁ mode light beam is a wavelength of 1,450 nm ormore.
 17. An optical fiber amplifier comprising: the optical fiber foramplification according to claim 1; and a pumping light sourceconfigured to emit a pumping light beam in a 980 nm wavelength band, thepumping light beam being entered to the core.